The present invention relates to a four-wheel drive motor vehicle of a type having a center differential between the front and rear wheels, and means for restraining or locking the center differential. More specifically, the present invention relates to a control system for restraining or locking a center differential of a four-wheel drive system in accordance with the deceleration of the vehicle.
A four-wheel drive vehicle is superior in ability for hill climbing and ability for rough road driving. If, however, all four wheels are always driven at the same speed, a four-wheel drive vehicle cannot be turned smoothly. During a turn with large steering angles of steerable wheels, the front wheel of an inner or outer side must travel along a circle having a large radius and the rear wheel of the same side must travel along a small circle. Because of this difference in turning radius, there arises a large difference between a rotation speed (an average rotation speed, to be exact) of the front wheels and a rotation speed (an average rotation speed) of the rear wheels. As a result, the steering becomes heavy, the tendency to understeer is increased, and the vehicle cannot be turned without abnormal tire friction (called tight corner brake) which tends to brake the vehicle and cause an engine stall.
To overcome these handling and tire friction problems, some four-wheel drive systems use a center differential between front and rear wheels. If, however, one of the four wheels runs on a muddy place, the center differential coacts with a rear or front differential between right and left wheels, so that the torque cannot be transmitted to the other three wheels, and the vehicle cannot escape from the muddy place.
In view of this problem, some four-wheel drive systems are further provided with means (a lockup mechanism or a non-slip differential mechanism) for restraining or preventing the action of the center differential. One example is shown in Japanese patent provisional publication No. 57-114727.
In a four-wheel drive vehicle of this type, when the action of the center differential is prevented by the center differential restraining or preventing means, the front wheels and rear wheels are forced to rotate integrally by the drive system, so that, during hard braking, the front wheels and rear wheels are forced to stop simultaneously. Because of this nature, four-wheel drive vehicles of this type can provide an excellent braking performance. However, the excellent braking performance can be obtained only in a driving mode in which the action of the center differential is prevented. It is very difficult or practically impossible for a driver to change the driving mode manually during hard braking.